Sediment size fractionation and focusing in the equatorial Pacific: Effect on 230Th normalization and paleoflux measurements

Abstract

We use flux, dissolution, and excess 230Th data from the Joint Global Ocean Flux Study and Manganese Nodule Project equatorial Pacific study Site C to assess the extent of sediment focusing in the equatorial Pacific. Measured mass accumulation rates (MAR) from sediment cores were compared to reconstructed MAR by multiplying the particulate rain caught in sediment traps by the 230Th focusing factor and subtracting measured dissolution. CaCO3 MAR is severely overestimated when the 230Th focusing factor correction is large but is estimated correctly when the focusing factor is small. In contrast, Al fluxes in the sediment fine fraction are well matched when the focusing correction is used. Since CaCO3 is primarily a coarse sediment component, we propose that there is significant sorting of fine and coarse sediments during lateral sediment transport by weak currents. Because CaCO3 does not move with 230Th, normalization typically overcorrects the CaCO3 MAR; and because CaCO3 is 80% of the total sediment, 230Th normalization overestimates lateral sediment flux. Fluxes of 230Th in particulate rain caught in sediment traps agree with the water column production-sorption model, except within 500m of the bottom. Near the bottom, 230Th flux measurements are as much as 3 times higher than model predictions. There is also evidence for lateral near-bottom 230Th transport in the bottom nepheloid layer since 230Th fluxes caught by near-bottom sediment traps are higher than predicted by resuspension of surface sediments alone. Resuspension and nepheloid layer transport under weak currents need to be better understood in order to use 230Th within a quantitative model of lateral sediment transport. © 2014. American Geophysical Union. All Rights Reserved.